1. Field of the Invention
This invention relates to coatings for steels and other metals which are exposed to bromide environments, which coatings reduce the corrosion rate of said steels and other metals in the bromide environment. More particularly, this invention relates to coatings for steels and other metals used in bromide-based absorption cycles which reduce the rate of corrosion of said metals. The coatings of this invention allow use of low-cost steels as a construction material in place of expensive alloys, allow an increase in operating temperature of such bromide-absorption cycles, up to about 300xc2x0 C., thereby enabling improvement in the coefficient of performance (COP), and result in extended service life, thereby reducing replacement and maintenance costs. Also disclosed is a method for coating such steels and other metals.
2. Description of Prior Art
Coatings are widely used for the corrosion protection of metals and other materials in a variety of environments. However, for each environment, different coatings may be required and, thus, careful selection and testing is necessary before a coating can be certified for a particular application.
The protection of steel in a variety of environments is the subject of many articles and patents. Although there are a large number of prior art patents and publications on corrosion in aqueous environments, it is not obvious from the prior art what coating should be selected for the protection of steel in a high temperature molten bromide medium in the presence of water such as one which is used in bromide-absorption cycles. A good example is the failure of titanium coatings in high temperature bromide media. Although the resistance of titanium to halide induced corrosion is well documented, we have found that titanium is readily attacked in high temperature bromide solutions. Similarly, Ti and other metals, such as V, Zr and their alloys, which are quoted often as corrosion resistant, may be attacked in the presence of sulfuric acid environments. Therefore, it is apparent that one cannot readily predict the suitability of a metal as a coating for materials exposed to bromide-based environments based upon the a general description of the corrosion resistance of the metal alone, nor can one readily predict the corrosion protection afforded by a given metal coating on a metal substrate.
An improved corrosion resistant surface layer on a metal substrate formed by laser-induced remelting and solidifying under an aqueous solution of metal ion and reducing agent is taught by Japanese Patent 61281856. Here, the surface layer is formed by a treatment involving remelting and solidifying the surface of the base material and consists of a number of minutely thin layers. The improved surface layer has a different composition from the base material. The method is used to impart a corrosion resistant layer to a metal, especially to stainless steel, for use, for example, in a nuclear fuel reprocessing plant, a chemical plant, nuclear power plant, absorption refrigerator, or in a semiconductor package. European Patent Publication 0488165 teaches the plating non-electrolytically, electrically, or by any other means, of copper and nickel on the surface of a copper heat transfer tube followed by diffusion of the copper and nickel, which is then work hardened by means of rolling, swaging or other known means. The procedure is applicable to any absorbing refrigerator using solutions such as an aqueous solution of lithium bromide or any other salt solution.
The diffusion coating of a metal by simultaneous deposition of Cr and Si onto the metal is taught by U.S. Pat. No. 5,492,727 and related U.S. Pat. No. 5,589,220. The method utilizes a halide-activated cementation pack with a dual halide activator. Codeposition of chromium and silicon coatings on iron-based alloys by pack cementation using a mixed activator, that is a fused salt solution of NaF and NaCl, is taught by U.S. Pat. No. 5,364,659.
A chemical vapor deposition (CVD) method for case hardening a ferrous metal interior tubular surface by exposure to diffusible boron with or without other diffusible elements such as silicon to enhance the wear, abrasion and corrosion resistance of the tubular surface is taught by U.S. Pat. No. 5,455,068. The use of chemical vapor deposition for deposit of aluminum and a metal oxide on substrates for improved corrosion, oxidation, and erosion protection is taught by U.S. Pat. No. 5,503,874.
A method for producing materials in the form of coatings or powders using a halogen-containing reactant which reacts with a second reactant to form one or more reactive intermediates from which the powder or coating may be formed by disproportionation, decomposition, or reaction is taught by U.S. Pat. No. 5,149,514.
U.S. Pat. No. 4,822,642 teaches a silicon diffusion coating formed in the surface of a metal article by exposing the metal article to a reducing atmosphere followed by treatment in an atmosphere of 1 ppm to 100% by volume silane, with the balance being hydrogen or hydrogen plus inert gas.
A method for depositing a hard metal alloy in which a volatile halide of titanium is reduced off the surface of a substrate and then reacted with a volatile halide of boron, carbon or silicon to effect the deposition on a substrate of an intermediate compound of titanium in a liquid phase is taught by U.S. Pat. No. 4,040,870.
The fact that the coatings disclosed by these references improve the corrosion resistance properties of a substrate material, even if only for a very short period of time, is obvious. However, the objective of any protective coating applied to a substrate metal is to provide corrosion protection properties to the substrate metal for extended periods of time when disposed in extremely aggressive environments such as bromide-based absorbents. In the case of bromide-based absorption systems, this means corrosion resistance properties better than the corrosion resistance properties of currently used expensive alloys, such as AL6XN(copyright) available from Allegheny Ludlum. The corrosion rate of AL6XN in bromide-based solutions is about 2.4 mils per year (mpy). By comparison, the corrosion rate of ANSI Type 409 stainless steel, a much less expensive metal than AL6XN, in a bromide-based solution is about 96 mpy. Thus, protective coatings which provide corrosion rates less than 96 mpy, more preferably less than 75 mpy, and most preferably less than about 2.4 mpy are required. None of the prior art references discussed herein provide any guidance with respect to coatings which, when applied to stainless steel substrates, provide the requisite corrosion resistant properties to the substrates when disposed in a bromide-based environment such as is encountered in bromide-based absorption systems.
Absorption cooling systems are heat driven refrigeration machines in which a secondary fluid, the absorbent, absorbs the primary fluid, gaseous refrigerant, that has been vaporized in an evaporator. In a typical single effect absorption refrigeration system, water is used as the refrigerant and lithium bromide as the absorbent. Other refrigerant/absorbent pairs (solutions) have been used, or have the potential for use, in absorption cycles.
In a single effect absorption chiller, refrigerant vapor is produced in an evaporator at a temperature somewhat below that of the heat load. The refrigerant vapor is exothermically absorbed by a concentrated absorbent solution entering the absorber. The heat of absorption is then transferred to a heat sink, such as cooling water, at the absorber. The dilute absorbent solution is then pumped to the generator, where it is concentrated again and returned to the absorber. External heat is supplied to the generator to supply the energy required to separate the refrigerant from the absorbent. The refrigerant is condensed at the condenser and is returned to the evaporator, while the concentrated absorbent is returned to the absorber. A heat exchanger between the absorber and generator is also part of the system, exchanging heat to the dilute absorbent from the concentrated absorbent solution. This process is carried out between two pressures, a lower pressure in the evaporator-absorber section and a higher pressure in the generator-condenser section. The operating temperature limits of the refrigerant/absorbent combinations are determined by the chemical and physical properties of the solution pair.
Advanced absorption systems fueled by natural gas may offer significant advantages over conventional heating, cooling, and refrigeration systems. These advanced systems include double effect, triple effect, and generator absorber heat exchange cycles. The advanced absorption systems reduce energy consumption, thus improving the economics of natural gas consumption. The reduced gas consumption per unit results in a lower operating cost to the consumer and lower emissions to the environment.
These advanced absorption systems transfer heat from both the absorber and condenser from a higher temperature absorption cycle using one refrigerant/absorbent pair to a second (or third) refrigerant/absorbent pair operating at a lower generator temperature. The triple effect chiller is 30% to 60% higher in coefficient of performance (COP) than a double effect cycle using equivalent heat exchangers. The triple effect chiller has the potential of being less expensive than double effect chillers because the triple effect chillers can use several existing absorption fluids. In addition, it uses only conventional heat exchangers and it requires less total heat exchanger per unit of capacity than single or double effect cycles. Water/LiBr, water/LiBr with additional absorbents, for example, ZnBr2, LiCl, ammonia/water, and ammonia/water with other salts, for example LiBr, are some of the widely used absorbents and refrigerants. Conventional absorbent cycles operate in the range of 200xc2x0 to 350xc2x0 F., but triple effect and other advanced cycles operate at much higher temperatures in order to transfer absorber heat at temperatures up to 300xc2x0 F. Because the condenser is also operating at 200xc2x0 F. as well as being under high pressure, the generator temperatures must be very high, typically greater than 450xc2x0 F. These very high temperatures result in very low concentrations of refrigerant in the liquid leaving the generator, which is extremely corrosive to the metals used in the construction of these heat exchangers.
The major barrier towards commercialization of advanced absorbent systems is the prohibitive cost of the construction material. The alloys that survive in salt mixtures at the extreme temperatures required for high COP are very expensive and difficult to machine. As a result, the use of efficient advanced absorption systems is not financially attractive.
Accordingly, it is an object of this invention to provide materials suitable for use in advanced absorption systems which are relatively low cost with respect to the expensive alloys currently required by advanced absorption systems, thereby eliminating the major barrier to the commercialization of such advanced absorption cycles.
It is another object of this invention to provide coated, low cost steel and other metals suitable for use in such advanced absorption cycles having the same or better corrosion resistance than the expensive alloys currently used.
It is yet another object of this invention to provide a method for producing such corrosion-resistant coated low cost steels and other metals.
These and other objects of this invention are addressed by a structure, such as a conduit or heat exchanger disposed in a bromide-based absorption system comprising stainless steel coated with a metallic coating comprising a metal selected from the group consisting of Cr, W, Si, Mo, Ti, Ni, Zr and mixtures thereof.
To produce the coated stainless steel of this invention, a metallic protective coating selected from the group consisting of Cr, W, Si, Mo, Ni, Ti, Zr and mixtures thereof is deposited onto a stainless steel substrate. The metallic protective coating is then diffused so as to form a diffusion layer between the metallic protective coating and the metal substrate. Optionally, the metallic protective coating may then be passivated, forming a stable compound selected from the group consisting of carbides, borides, nitrides, silicides, oxides and mixtures thereof on the metallic protective coating. Suitable deposition methods for deposition of the metallic protective coating on the metal substrate include chemical vapor deposition, pack cementation, and a fluidized bed reactor. The preferred means of deposition is by way of a fluidized bed reactor.
One of the benefits of this method for coating stainless steel and other metals is that it can be applied to a variety of convoluted geometries such as heat exchangers or bundles of tubes producing, for example, a heat exchanger suitable for use in a bromide-based absorption cycle wherein the heat exchanger comprises a metallic protective coating selected from the group consisting of Si, Cr, W, Mo, Ni, Ti, Zr and mixtures thereof, and a diffusion layer formed by diffusion of the metallic protective coating disposed between the substrate metal and the metallic protective coating.